Strings

1. Strings CSE 1310 – Introduction to Computers and Programming Vassilis Athitsos University of Texas at Arlington

2. String Comparisons >>> my_strings = ["Welcome", "to", "the", "city", "of", "New", "York"] >>> my_strings ['Welcome', 'to', 'the', 'city', 'of', 'New', 'York'] >>> my_strings.sort() >>> my_strings ['New', 'Welcome', 'York', 'city', 'of', 'the', 'to'] • Python uses a string order of its own. • Follows alphabetical order, with the exception that capital letters are always before lower case letters.

3. String Comparisons • It is easy to verify the order that Python uses, by trying out different pairs of strings. >>> "hello" < "goodbye" False >>> "Hello" < "goodbye" True >>> "ab" > "abc" False

4. String Comparisons >>> "123" < "abc" True >>> "123" < "ABC" True • Numbers come before letters. • Guideline: do not memorize these rules, just remember that Python does NOT use exact alphabetical order.

5. Strings Cannot Change >>> a = "Munday" >>> a[1] = 'o' Traceback (most recent call last): File "<pyshell#297>", line 1, in <module> a[1] = 'o' TypeError: 'str' object does not support item assignment

6. If You Must Change a String… • You cannot, but you can make your variable equal to another string that is what you want. • Example: >>> my_string = "Munday" • my_string contains a value that we want to correct. >>> my_string = "Monday" • We just assign to variable my_string a new string value, that is what we want.

7. For More Subtle String Changes… • Suppose that we want a program that: • Gets a string from the user. • Replaces the third letter of that string with the letter A. • Prints out the modified string. We just assign to variable my_string a new string value, that is what we want.

8. For More Subtle String Changes… • Strategy: • convert string to list of characters • do any manipulations we want to the list (since lists can change) • convert list of characters back to a string

9. An Example • Write a program that: • Gets a string from the user. • Modifies that string so that position 3 is an A. • Prints the modified string.

10. An Example my_string = raw_input("please enter a string: ") if (len(my_string) >= 3): # convert string to list, make the desired change (change third letter to "A") my_list = list(my_string) my_list[2] = "A"; # create a string out of the characters in the list new_string = "" for character in my_list: new_string = new_string + character my_string = new_string print "the modified string is", my_string

11. A Variation my_string = raw_input("please enter a string: ") my_string = my_string[0:2] + "A" + my_string[3:] print "the modified string is", my_string

12. The * Operator on Strings >>> a = "hello" >>> a*3 'hellohellohello' • The string*integer expression repeats a string as many times as the integer specifies.

13. The in Operator • The in operator works for lists and strings. • Syntax: • element in container • Returns true if the element appears in the container, false otherwise. >>> a = [1, 2, 3] >>> 2 in a True >>> 5 in a False >>> vowels = 'aeiou' >>> "a" in vowels True >>> "k" in vowels False

14. upper and lower • The string.upper() method returns a new string where all letters are upper case. • The string.lower() method returns a new string where all letters are lower case. • Note: upper() and lower() do not modify the original string, they just create a new string. • Should be obvious, because strings cannot be modified. >>> vowels = 'aeiou' >>> b = vowels.upper() >>> vowels 'aeiou' >>> b 'AEIOU' >>> a = 'New York City' >>> b = a.lower() >>> b 'new york city'

15. The lenFunction • Similar as in lists, len returns the number of letters in a string. >>> len('hello') 5

16. Reversing a String >>> a = "hello" >>> b = a[::-1] >>> b 'olleh' >>> a[3:0:-1] 'lle' • Slicing with step -1 can be used to reverse parts, or all of the string.

17. index and find • The my_list.index(X) method returns the first position where X occurs. • Gives an error if X is not in my_list. • The my_string.find(X) method returns the first position where X occurs. • X can be a single letter or more letters. • Returns -1 if X is not found. >>> a = [10, 11, 12, 10, 11] >>> a.index(10) 0 >>> a.index(11) 1 >>> b = "this is crazy" >>> b.find('is') 2 >>> b.find('q') -1

18. Converting Other Types to Strings • The str function converts objects of other types into strings. • Note:str does NOT concatenate a list of characters (or strings). See example on left. >>> a = 2012 >>> b = str(a) >>> b '2012' >>> a = ['h', 'e', 'l', 'l', 'o'] >>> b = str(a) >>> b "['h', 'e', 'l', 'l', 'o']"

19. Converting Strings Into Ints/Floats • The int, float functions converts strings to integers and floats. • Will give error message if the string does not represent an integer or float. >>> a = '2012' >>> b = int(a) >>> b 2012 >>> float(a) 2012.0 >>> a = "57 bus" >>> int(a) <error message>

20. Converting Strings Into Lists • The list function can convert a string to a list. • Always works. • Very handy if we want to manipulate a string's contents and create new strings based on them. >>> a = "hello" >>> list(a) ['h', 'e', 'l', 'l', 'o']